1. Introduction

Low-loss subwavelength-width waveguides are essential elements for ultra-compact photonic circuits and devices. Recent works have shown that optical wave guiding with taper-drawn silica nanowires or chemically grown nanoribbons and/or nanowires are promising approaches for circumventing precision limitations of conventional fabrication methods [1–7]. Generally, physical drawing techniques yield amorphous nanowires with optical losses of less than 0.1 dB/mm but are limited to materials used in traditional optical fibers [1,5,8,9], whereas chemical growth methods produce versatile nanoribbons, albeit with losses higher than 1 dB/mm [3,4,6]. Here we show that low-loss versatile photonic nanowires can be directly drawn from bulk glasses, and it is not necessary to pull glasses into standard fibers before drawing nanowires. Fabricated by local-melting and drawing of functionalized glasses, these nanowires not only show low loss for optical wave guiding, but also offer favorable properties such as high index for tight optical confinement and photoluminescence when doped with rare earth ions. The capability of directly drawing photonic nanowires from glasses may open up vast opportunities for making versatile building blocks for micro- and nanoscale photonic circuits and components, as well as for functionalizing photonic glasses on nanometer scale.

2. Direct draw of photonic nanowires from bulk glasses

Our approach for directly drawing glass nanowires is illustrated in Fig .1. First, we use a CO₂ laser or flame to heat a sapphire fiber (400–700 μm in diameter) to a temperature high enough for melting the glass, and move the glass towards the fiber. When the fiber immerses into the glass through local melting, we withdraw the glass with a portion of melt left on the fiber. We then bring a second sapphire fiber (about 400 μm in diameter) into contact with the glass-coated sapphire fiber end, reduce or remove the heating power to cool down the melt to a proper temperature (e.g. 800–1000 K for phosphate glass), and withdraw the second sapphire fiber at a speed of 0.1–1m/s to draw wires from the melt until breakage of the wire. When the process is finished, a nanowire with considerable length is formed at the freestanding side of the taper drawn wire. An argon gas atmosphere with laser heating is applied for drawing nanowires from chemically unstable materials such as fluoride glasses. This technique can also be applied to pulverized glasses, allowing for adjustments to the properties of the nanowire by tailoring the composition of the initial powder. In addition, nanowires can be drawn using extremely low starting quantities of glass (e.g. 1 mg in mass), greatly reducing the quantity of starting material required.

 

Fig. 1. Schematic diagram illustrating the direct draw of nanowires from bulk glasses. (1) A glass is moved towards a sapphire fiber heated by a CO₂ laser or flame. (2) The fiber end is immersed into the glass through local melting. (3) A portion of molten glass is left on the end of the fiber when the glass is withdrawn. (4) A second sapphire fiber is brought into contact with the molten-glass-coated end of the first sapphire fiber. (5) The heating power is reduced and the second sapphire fiber is withdrawn. (6) A nanowire is formed at the freestanding side of the taper drawn wire.

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With the technique shown in Fig. 1, a series of glass materials such as phosphate, fluoride, silicate and tellurite glasses have been drawn into highly uniform nanowires with diameters down to 50 nm and lengths up to tens of millimetres. Figure 2(a) shows a scanning electron microscope (SEM) image of a 100-nm-diameter tellurite glass (70TeO₂-25ZnO-5La₂O₃) nanowire. The uniform diameter and defect-free surface of the wire is clearly visible. For reference, measured diameter variation ΔD is less than 10 nm over a 20-μm length L of the wire, giving the diameter uniformity ΔD/L better than 5×10^-4. The high uniformity and integrity give the wires considerable strength and pliability for manipulation. For example, Fig. 2(b) shows an elastically bent 320-nm-diameter silicate glass (Corning 0215) nanowire with a minimum bending radius of 5 μm, the wire shows a tensile strength sufficient for withstanding such a sharp bend. Fig. 2(c) gives an SEM image of the fracture face of a 400-nm-diameter tellurite glass nanowire, showing the high-symmetric circular cross section of the wire. Such a perfect circular cross section can be explained by the surface tension and amorphous nature of the glass. The as-drawn nanowires can also be plastically bent with an annealing-after-bending procedure [5]. For reference, Figs. 2(d) and 2(e) show a spiral plastic bend of an 80-nm-diameter Er and Yb co-doped phosphate glass (65P₂O₅-12Al₂O₃-5Li₂O-16Na₂O-1.5Yb2O₃-0.5Er₂O₃) nanowire and sharp turns made on a 170-nm-diameter tellurite glass (70TeO₂-25ZnO-5La₂O₃) nanowire. The surface roughness of the nanowire is examined using a transmission electron microscope (TEM). Figure 2(f) shows a TEM image of the sidewall of a 210-nm-diameter Er and Yb co-doped phosphate glass (65P₂O₅-12Al₂O₃-5Li₂O-16Na₂O-1.5Yb₂O₃-0.5Er₂O₃) nanowire, showing no visible defects or irregularities on the wire surface. Typical sidewall root-mean-square roughness of these nanowires is around 0.3 nm, which is on the same order of silica nanowires drawn from optical fibers and approaching the intrinsic roughness of melt formed glass surfaces [10–12]. These nanowires have excellent diameter uniformity, defect-free surface, high strength and hospitality to ion dopants, as well as the low dimension for single-mode operation (see Fig. 3(a)), making them ideal for photonic applications.

 

Fig. 2. Electron microscopic characterizations of as-drawn glass nanowires. (a) SEM image of a 100-nm-diameter tellurite glass nanowire. (b) SEM image of an elastically bent 320-nm-diameter silicate glass nanowire. (c) SEM image of the cross section of a 400-nm-diameter tellurite glass nanowire. (d) SEM image of a spiral plastic bend of an 80-nm-diameter phosphate glass nanowire. (e) SEM image of a 170-nm-diameter tellurite glass nanowire with sharp plastic bends. (f) TEM examination of the sidewall of a 210-nm-diameter phosphate glass nanowire.

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3. Optical properties and potentials of photonic nanowires

For photonic applications, coupling light in and out of a nanowire is an essential step. In previous work [1,5], evanescently coupling light into a silica nanowire using an optical fiber tapered down to a uniform nanowire has proven to be an efficient method. However, the nanowires obtained here are drawn from different glasses that may show a large index difference with the silica nanotaper, resulting in large differences in propagation constants (see Fig. 3(a), obtained by numerical calculations [13,14]) that may hinder efficient coupling. Here we use a nanotaper with a steeply tapered shape to account for this difference. As shown in Fig. 3(b), a uniform nanowire is brought into contact with a nanotaper that is tapered down from an optical fiber. Unlike nanotapers used in previous works [1,5], this taper maintains an obvious tapering tendency along its whole length, making it possible to match the effective index of the nanotaper to that of the nanowire within the coupling region. Using this scheme, effective coupling (e.g. coupling efficiency of 80% in phosphate glass nanowires) is obtained.

 

Fig. 3. Launching light into a single nanowire. (a) Calculated propagation constants for glass nanowires with refractive indices of 1.46 (silica), 1.48 (fluoride), 1.54 (phosphate), 1.89 (germinate) and 2.02 (tellurite) respectively. A circle marked on each curve locates single-mode cut-off diameter of the nanowire. (b) Schematic diagram of launching light into a single nanowire by evanescent coupling.

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Optical properties of glass nanowires are investigated by evanescently coupling light into a single nanowire and measuring its output a certain distance away. Figure 4(a) gives the optical guiding losses of four types of nanowires at 633-nm wavelength: freestanding phosphate (n =1.54), silicate (n =1.52) and tellurite (n =2.02) glass nanowires, and MgF₂ (n =1.39) supported tellurite glass nanowires. All the nanowires show guiding losses lower than 0.1 dB/mm at their single-mode cut-off diameters (e.g. 280 nm for tellurite, 410 nm for phosphate and 420 nm for silicate glass nanowires), about one or two orders of magnitude lower than the loss reported in chemically grown nanoribbons and nanowires [3,4,6]. With smaller diameters (e.g. 200 nm), the tellurite glass nanowires show lower loss than other wires, which can be explained by lower scattering loss in the high-index nanowire due to tighter optical confinement. In addition, optical loss in MgF₂-supported tellurite glass nanowires is very close to that of freestanding wires, indicating that substrate-induced loss, deemed to be the dominant loss factor in chemically grown nanoribbons [3,4], is almost negligible in the physically drawn nanowires demonstrated here. For reference, Fig. 4(b) shows an optical microscope image of a MgF₂-crystal-supported 260-nm-diameter tellurite glass nanowire; the wire guides 633-nm-wavelength light on the surface of the MgF₂ and transmits light to the top right end. No scattering is observed along the whole length of the nanowire in spite of the strong guided intensity, indicating the low optical loss and no substrate-induced loss of the wire.

 

Fig. 4. Optical investigation of glass nanowires. The white arrow in (b), (c) and (f) indicates the direction of light propagation. (a) Measured loss of freestanding phosphate, silicate, tellurite, and MgF₂-supported tellurite glass nanowires. (b) Optical micrograph of a 260-nm-diameter tellurite glass nanowire guiding 633-nm-wavelength light on the surface of a MgF₂ crystal. (c) Photoluminescence image of a 320-nm-diameter 0.1 mol% Er-doped ZBLAN nanowire excited by a 975-nm-wavelength light coming from the nanotaper on the left-hand side. The up-conversion luminescence (green light) is clearly visible. A second nanotaper at the right-hand side picks up the luminescence for spectral measurement with results shown in (d). (e) Emission spectrum of a 510-nm-diameter Er and Yb co-doped phosphate glass nanowire excited at 975-nm wavelength. (f) Optical micrograph of an optical coupler assembled using two tellurite glass nanowires (350 and 450 nm in diameter respectively) on the surface of a silicate glass. The coupler splits the 633-nm-wavelength light equally.

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Photoluminescence is observed in nanowires drawn from rare earth doped glasses when they are properly excited. Shown in Fig. 4(c) is a photoluminescence image of a 320-nm-diameter 0.1 mol% Er-doped ZBLAN (53ZrF₄-20BaF₂-3.9LaF₃-3AlF₃-20NaF-0.1ErF₃) glass nanowire excited at 975-nm wavelength. The pumping light is evanescently coupled into the nanowire by a fiber nanotaper. The strong up-conversion green luminescence is clearly seen and is picked up by another nanotaper about 100 μm along the wire. The emission spectrum given in Fig. 4(d) shows strong emission peaks centred around 550, 658 and 848 nm, which can be assigned to the transitions between ²H_11/2, ⁴F_9/2, ⁴I_9/2 and ground state ²I_15/2 of Er³⁺ ions, respectively. Using similar schemes, photoluminescence from other nanowires was observed. Figure 4(e), for example, shows broad-band fluorescent emission from a 510-nm-diameter Er and Yb co-doped phosphate glass (63P₂O₅-10Al₂O₃-17Li₂O-8Yb₂O₃-0.05Er₂O₃-2La₂O₃) nanowire (excited at 975-nm wavelength) centred around 1015 and 1535 nm. These phenomena open up the possibilities of building active devices using doped glass nanowires. For example, when incorporated with high-Q ring resonators [15–17], doped nanowires may be developed into nanowire ring lasers, as well as nanowire optical sensors when the photoluminescence is used as a built-in light source [6].

Compared with fiber-drawn silica nanowires [1,18–21], nanowires drawn from high refractive index glasses such as tellurite and bismuth-based glasses offer the opportunity of much tighter optical confinement. Numerical calculations show that [14], at the wavelength of 633 nm, a 280-nm-diameter silica nanowire (n =1.46) confines only 36% of the guided light inside the core, while a tellurite nanowire (n =2.02) of the same diameter offers a confinement higher than 80%. Tight confinement is essential for guiding light through sharp turns with low radiation loss and reducing cross-talk between adjacent waveguides, which are particularly desirable for miniaturization and high-density integration of optical waveguiding structures and components [22–24].

The high refractive index of nanowires also allows to supporting these nanowires with common substrates such as silicate glass and MgF₂ crystal. Figure 4(f) shows an optical coupler assembled using two tellurite glass (70TeO₂-25ZnO-5La₂O₃) nanowires supported by a silica substrate with a refractive index of 1.46. The diameters of the two nanowires are 350 and 450 nm respectively. When 633-nm-wavelength light is launched into the bottom left arm, the coupler splits the flow of light in two. Because of the large index contrast between the tellurite (2.02) and silica (1.45) glass, the light is well confined on the surface of the substrate and guided along the nanowires. For comparison, low-index silica nanowires with similar geometries require ultra-low-index porous substrates such as silica aerogel that is usually inconvenient for handling and mechanically weak [5]. Furthermore, a higher index contrast and a solid substrate make it possible to build devices with lower loss and smaller sizes. As shown in Fig. 4(f), the wire assembly works as a 3-dB coupler (50/50 splitter in this case) with virtually no excess loss (no scattering is observed around the coupling area). Also, the overlap length (~ 4 μm) is much shorter than that required by microcouplers fabricated with conventional methods [25].

4. Discussions and conclusions

In summary, we have demonstrated the direct draw of photonic nanowires from bulk glasses instead of glass fibers, which . As-drawn nanowires not only show low optical losses, but also offer favorable properties such as high index for tight optical confinement and photoluminescence for active devices, making them very promising building blocks for future micro- and nanoscale photonic circuits and devices. As one of the fundamental materials in fields including optics, electronics, chemistry and biology, glass has a number of advantages over other materials (e.g. crystal) in homogeneity, transparency, ease of fabrication and excellent solvent properties [26–28], which together with the low dimension of the nanowire should inspire broad interest in a variety of fields. Moreover, the technique for directly drawing nanowires from glasses, as well as the capability of effectively coupling light in and out of a single wire, may present new opportunities for exploring glasses on the nanometer scale.